The present invention relates to a polarity-alternated pulse width code division multiple access (PAPW/CDMA) conceived to resolve the inherent problems of the conventional direct sequence (DS)/CDMA that modulated output has the multiple level; i.e., the present invention is capable of enhancing power efficiency of a system by controlling a transmitted modulation waveform to have a binary form as in time division multiple access (TDMA), which necessitates neither an RF amplifier requiring linearity at a transmitting terminal nor an A/D converter to process binary signals at a receiving terminal. In particular, the present invention relates to a method for preventing deterioration of the system performance due to a certain data pattern by applying a mask pattern to input data. The present invention also relates to a method for measuring a distance between communicating equipments using a polarity alternated pulse width CDMA. The technology of multiplexing for wireless transmission is classified into three methods: a frequency division multiple access (FDMA); a TDMA; and a CDMA. Of those, the CDMA has been chosen as an international standard in the IMT-2000 system, which is referred to as the third generation mobile communications, thank to its superiority in general characteristics despite the complexity in its realization, after a fierce competition with TDMA in the second generation mobile communications.
The CDMA, a multiple access method based on the technology of a spread-spectrum system, is referred to as a spread-spectrum multiple access (SSMA) method.
From the historical perspective, the spread-spectrum principle has been developed during the outbreak of the 1st World War and the 2nd World War. Spread-spectrum technology originated from the concept that interference can be overcome by spreading the spectrum of signal has become a subject of intense research since 1950s to be mainly used for military communications and satellite communications. With the recent evolution of the semiconductor technology in integrity and operating speed as well as the microprocessor technology, the spread-spectrum system is used even in the commercial mobile communications in addition to the military communications and satellite communications.
The spread-spectrum system is basically classified into three kinds depending on the methods for spreading the spectrum of signal: direct-sequence (DS) spread spectrum, frequency-hopped (FH) spread spectrum, and time-hopped (TH) spread spectrum.
Among them, the CDMA system using a DS spread spectrum adopts a manner of spreading the spectrum of signal by directly modulating, i.e., by multiplying the data sequence carrier by a broadband spreading signal. The spreading signal composed of +1 and −1 is referred to as spreading sequence, spreading code or code sequence. A transmitted spreading signal arrives at a receiver in a form distorted by noise, interference or signal disturbance, etc. The receiver multiplies the received signal by the spread signal that is identical to that used by a transmitter to de-spread the received signal spectrum of which is spread.
However, the conventional DS/CDMA simultaneously transmits signals of multiple channels through linear summation. Thus, an increase in the number of channels results in an increase in the number of the output signal levels, and the amplitude subsequently becomes variable as in case of analog signals. Therefore, a linear amplifier has to be used to amplify a high frequency to maintain the linearity of modulated signal at the transmitter; the signal has to be processed as a multi-bit after digitizing through the analog/digital converter at the receiver. This is very complicated process. Such complexity of the DS/CDMA is expected to result in difficulties in designing and realizing the system as the significance of the mobile communications increases and the transmission rate of wireless communications goes higher in the future.
FIG. 1 is a block diagram illustrating a module of a conventional DS/CDMA transmitter.
Referring to FIG. 1, input signals d1, d2, . . . , dn are multiplied by orthogonal code patterns c1, c2, . . . , cn by means of multipliers 1a, 1b, . . . , 1n, respectively. The multiplied signals are summed up by an adder 10 to form an analog signal {s} of n+1 levels and to be transferred to a radio frequency (RF) amplifier (not shown in the figure). In the conventional DS/CDMA, it becomes complicated to maintain the linearity of signal characteristics such that a linear amplifier has to be used to amplify a high frequency to maintain the linearity of modulated signal at the transmitter; the signal has to be processed as a multi-bit after digitizing through the analog/digital converter at the receiver, as mentioned above.